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Limits of detection — Enhancing identification of anthropogenic radionuclides
Nuclear Inst. and Methods in Physics Research, A 947 (2019) 162818
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Limits of detection — Enhancing identification of anthropogenic radionuclides R. Britton ∗, A.V. Davies AWE, Aldermaston, Reading, Berkshire, RG7 4PR, UK
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Keywords: Coincidence spectrometry Gamma spectroscopy List-mode Detection limits CTBT
ABSTRACT The sensitivity of 𝛾-spectrometry is defined by three fundamental factors — the particulars of each isotopes decay mechanism, the losses in the detector system, and the background radiation present during the measurement. This ‘glass floor’ therefore differs for each nuclide, but has the latter two components in common. Conventional measurements aim to minimise these common components, and therefore improve sensitivity for all isotopes. This research discusses the current (operational) sensitivity levels for 𝛾-spectrometry, and how new systems can improve upon these by a factor of 103 –106 . Finally, the ultimate sensitivity achievable is considered, along with practical considerations and potential improvements to get closer to this limit.
1. Introduction Gamma (𝛾) spectrometry is used in a diverse range of fields, from food safety studies to the verification of nuclear material. The limit of its application lies in two key considerations — the presence of a radionuclide that emits 𝛾 radiation in the sample to be measured, and the sensitivity of the detection systems available to laboratories. At GBL15 (the UK Comprehensive Nuclear-Test-Ban Treaty measurement facility based at AWE, Aldermaston), traditional 𝛾-spectrometry has been in use for decades. GBL15 now supports a number of international non-proliferation and treaty verification efforts. As part of these efforts, the laboratory has been focusing on developing revolutionary technologies that can provide a sea-change in the sensitivity achievable. Substantial research efforts have been invested in coincidence based systems [1,2], deep-underground systems, and the operational analysis of complex datasets [3–6]. During these efforts, it has become clear that 𝛾-spectrometry has not yet approached its ultimate limit, and pushing the current limits may yield a number of new applications for what is already an incredibly useful non-destructive analytical method; this paper considers both the advances made to date and the potential gains to be made in the future. 2. Traditional 𝜸-spectrometry High-resolution 𝛾-spectrometry relies heavily on High Purity Germanium (HPGe) based detectors, as no other semi-conductor or scintillation based device can match the performance (due to the combination of efficiency and resolution) achievable with these systems. Samples often contain hundreds of peaks, which would be unresolvable with
Fig. 1. A typical detector system undergoing testing prior to deployment at RN67, the CTBT International Monitoring System (IMS) station on St Helena. A single crystal detector is shielded with copper, tin, and lead. The entire shield is enclosed in BC408 plastic scintillation plates to veto cosmic induced events in the HPGe detector.
any scintillation based device (such as NaI(Tl)). Similarly, other semiconductor based devices, such as silicon detectors, cannot yet be made with large enough crystals to have a substantial efficiency above a few hundred keV. In the field of HPGe based 𝛾-spectrometry, crystals have become bigger, and recent advances in the electrical contacts have allowed greater resolution for planar crystals and well detectors (such as the Small Anode Germanium – SAGe – well designs from Mirion [7]). Electronics have also improved, with fully digitised Multi-Channel Analysers (MCAs) now the norm. Fundamentally, however, a 𝛾-spectrometer
∗ Corresponding author. E-mail address: [email protected] (R. Britton).
https://doi.org/10.1016/j.nima.2019.162818 Received 16 August 2019; Received in revised form 17 September 2019; Accepted 18 September 2019 Available online 21 September 2019 0168-9002/Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved.
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looks the same as 20 years previous — a single detector HPGe crystal surrounded with lead to reduce the terrestrial background, possibly with a plastic scintillation plate to veto cosmic radiation contributions to the background [8], see Fig. 1. In a surface based laboratory (such as GBL15), the HPGe detectors used are typically the largest that are commercially available, and built using materials specifically selected for a low radiation background. Background is further reduced using a shield of 100 mm of lead (containing a small percentage of antimony), and another 50 mm of aged lead inside of this. Liners of tin, cadmium, and copper are often used to attenuate X-rays, and a BC408 plastic scintillation plate (typically 1 m2 ) is used to suppress cosmic radiation induced events in the crystal [9]. With all of these techniques, sensitivity is limited to around the 10 μBq/m−3 level for 140 Ba in a high-volume air filter that is counted for a week. Given a typical air collection volume of ∼20000 m3 in 24 h, this equates to a specific activity of ∼0.2 Bq in the sample. During the analysis, single peaks are used for the quantification of radionuclides (typically those with the highest emission probability) with other peaks used if present to calculate a weighted mean for the activity level. It is worth noting that the losses in the signal collection/analysis chain are around a factor of 100, based on a 10% branching ratio, and (a somewhat optimistic) 10% peak efficiency.
Fig. 2. The research system at GBL15, with a large internal cavity to minimise backscatter of 𝛾’s from the lead into the detectors. It is built in a modular fashion, and as such can include a number of NaI detectors to act as a Compton shield, and silicon pin based 𝛽 cells. The inset shows the power supplies for the coolers, a LYNX™ MCA per detector, and the HEXAGON™ MCA.
3. Enhanced 𝜸-spectrometry 3.1. The detector system One way to improve the detection sensitivity is to increase the efficiency. Well type detectors are excellent in this regard, however suffer acutely from cascade summing effects [7]. This is where a cascade of 𝛾 radiation (which is fairly typical of how most nuclei decay) is captured in the detector, creating a signal with an amplitude that is somewhere between 0 keV and the total energy of the cascade. Given the variability in this signal, it cannot currently be reliably used for nuclide identification/quantification. Given that the overriding aim of all research systems at GBL15 is to improve operational capabilities, simplicity is key. The simplest solution that circumvents this issue is to include a second detector, as this doubles the detection efficiency and does not increase the systems sensitivity to cascade summing. Crucially, it also allows the detection and identification of 𝛾 cascades, which can provide an often unique signature that is far more distinctive than the single peaks used in traditional 𝛾 spectrometry. The research system at GBL15 consists of two electrically cooled BEGe 6530 detectors from Mirion. Multiple data acquisition chains are used, including LYNX™ MCAs from Mirion and a CAEN S.p.A HEXAGON™ dual-channel MCA. The detectors are situated in a faceto-face configuration, and enclosed in 150 mm lead with a retractable lid mounted on a rack-and-pinion system. Four 1 m2 BC408 plastic scintillation plates sit atop of the lid, providing suppression of cosmic events. All data is collected in time-stamped list-mode, with channels in each electronics chain time-synchronised (the clock bases differ for the two electronics chains — the LYNX™ units have a 100 MHz clock, whereas the HEXAGON™ has a 1 GHz clock). Up to 6 NaI detectors can also be inserted to provide Compton suppression during both traditional and enhanced operation, however none were in place for the analyses presented within this work (see Fig. 2).
Fig. 3. A traditional spectrum (blue) collected from a fission product sample measured on a HPGe detector. The total spectrum from all coincidences (𝛾 cascades) is shown in red. By requiring that one detector measures the 1596 keV 𝛾 from 140 La, the gated coincidence spectrum (green) results in an extremely clean nuclide signature, with almost all identified peaks arising from 140 La. All other nuclides (which are effectively background when characterising the 140 La signature) are dramatically reduced by a factor of 103 –104 . Note that this data can be sliced multiple times, creating clean coincidence projections for each nuclide of interest. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
loss in coincidence mode is therefore from the extra efficiency term of the second detector (assuming this also is around 10%, the total loss in the analysis chain is now around 1000). Whilst this is an order of magnitude more than in a traditional system, the selectivity available allows the background to be reduced from thousands of counts to just a few in the region of interest, and for very clean nuclide signatures to be obtained (see Fig. 3). To extract these signatures, GBL15 have developed C++/ROOT [10] based software known as the ‘AnalysisPackage’. This sorts all list-mode data into ROOT trees, extracts coincidences based upon variable timing windows, and builds both the histograms and matrices required for efficient analysis. The AnalysisPackage can gain-match and calibrate these data structures, and also includes the functionality to create advanced energy and time projections based upon multiple gates, fitting peaks/decay curves to the resulting projections as appropriate. To calculate the factors required to quantify these signatures, GBL15 use RIMMER [2,3], which only requires accurate peak/total efficiency characterisations and the appropriate ENSDF data [11] to calculate quantification factors for any single detector or coincidence based system.
3.2. Analysis Analysis of data from such a system is more complex than with a traditional system, however not prohibitively so. Data can simply be summed to create a detector with twice the efficiency, or events timecorrelated to analyse the cascades of 𝛾 radiation emitted. Whilst these cascades are isotope dependent, many have similar probabilities to the single emissions (within an order of magnitude). The only additional
4. Maximising detection sensitivity To quantify the detection limits possible with the research system, a 30 day acquisition was undertaken with a real sample. This 2
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on its own. Adding a third region (the 569.3 keV) reduces the sensitivity to ∼ 3.2 × 10−7 Bq m−3 due to the lower detectability of this emission when compared to the other 𝛾 signatures (the background in all three regions is similar). Identifying which combinations are optimal is inherently complex, as this cannot be calculated without the final histogram/matrix (which are used to determine the background, either directly from the gainmatched sum spectrum or from the sum of the projected coincidence spectra). Using the output of RIMMER (which contains the top twenty detectable signatures for both traditional and enhanced analysis for all radionuclides of interest) in combination with the ROOT file created by the AnalysisPackage, a ‘brute-force’ approach is used to evaluate all possible combinations of signatures and find the optimal subset. This process has been highly automated using a small PyROOT based application, known in GBL15 as MUGS (MUlti-Gamma Surveyor). This imports the RIMMER data, and uses it to identify regions and calculate background integrals for every emission in the RIMMER library. MUGS then moves onto the coincidence matrix, creating projections for each 𝛾-energy pair and evaluating the resulting integral in the region of interest. Finally, MUGS calculates the MDC for each signature on its own, followed by every possible combination of signatures that could be used for each nuclide. The analyst is then presented with the top three signatures (along with the MDC) for traditional, enhanced, and multi-signature analysis. It is worth noting that in practice, it is unlikely for many emissions to combine and provide a vastly more sensitive result than a single signature alone. Typically, one or two dominant signatures will provide the best result, and this is (of course) dependent on the sample being measured. The entire process (including the processing of 30 days of listmode data using the AnalysisPackage, running RIMMER to generate the quantification factors and 𝛾-cascade detection probabilities, and then using MUGS to calculate and identify the optimal signatures for each methodology and nuclide) takes less than a minute.
was from eastern Europe, and was one sample in a series of collections that detected 106 Rh during September and October 2017. Fig. 4 shows the measurement results using both traditional and enhanced methodologies. In Fig. 4, clear signals are seen from 106 Rh, which is a daughter of the 106 Ru that was collected on the air filter. To calculate the sensitivity levels for other radionuclides, the level of background can be used in each region of interest to calculate a critical limit, either based on the Currie method (where the number of counts in the region is great enough for a Gaussian approximation to be valid) or directly using Poisson statistics. The regions are automatically determined using RIMMER, which (in this study) is configured to provide a list of the top 20 signatures for each methodology and nuclide with the highest detectability. The sample was measured in March/April 2018, and all sensitivity levels are decay corrected to the start of acquisition. 4.1. An example with
134 Cs
4.1.1. Traditional 𝛾-spectrometry Using a single detector, a peak efficiency of 4.3% for the 604.72 keV emission, an abundance of 97.6% and a cascade summing correction of 0.806 (both calculated using RIMMER), the total loss factor is ∼29; i.e. the number of counts in this peak (if 134 Cs was present in the sample) would be 29 times lower than the number of decays that occurred. For both detectors, the loss factor is obviously half of this. There are 25776 background counts in this peak region – defined as the FWHM (Full-Width at Half-Maximum) of the detector at this energy multiplied by 1.25 – which results in a critical limit of 749. This is the number of counts required for statistical significance, and when combined with the loss factor, requires the total number of decays from 134 Cs in the sample to exceed 20851 during the measurement for confirmation of the detection to occur. Given that the total count time was 2431920 s, and the collection volume 25194 m3 , the Minimum Detectable Concentration (MDC) is ∼ 3.6 × 10−7 Bq m−3 (or ∼ 2.5 × 10−7 Bq m−3 for both detectors).
4.2. Virtual peaks and false-positives
4.1.2. Enhanced 𝛾-spectrometry Using the coincidence detection capabilities of the full system, a number of emissions can be included to contribute to the signal; the 795.9 keV emission is detected in coincidence with both the 604.7 keV emission 0.246% of the time, and the 569.3 keV emission 0.0326% of the time (calculated using RIMMER). Using the AnalysisPackage to gate on both of these signals, the total 795.9 keV peak can be projected, creating a signal with a total loss factor of ∼296 (note that separately, the loss factors would be ∼406 and ∼3067 respectively). Despite the combined loss factor being ∼1 order of magnitude higher than the traditional measurement, the background in this region, as a result of both projections, is 0. This equates to a critical limit of 3, and therefore the total number of decays required for detection to exceed 1078 during the measurement. The MDC is therefore ∼ 1.8 × 10−8 Bq m−3 , ∼10% lower than when using a single coincidence pair, and a factor of ∼20 lower than when using the most effective single emission (604.7 keV).
When going beyond a mere sensitivity study (and attempting to detect a nuclide) one must consider how to combine these signatures in the analysis. For coincidence pairs with a common energy, this is trivial, as gating on the non-common energy will result in perfectly aligned projections. When the energies are different, the analyst must consider the idea of a ‘Virtual Peak’. Here, the analysis regions identified using MUGS must be carefully normalised and re-binned (preferably to the lowest energy peak) so that they can be recombined. This recombination must account for both the potential variability in the binning of the original signal, and the compressing of the region to the reduced FWHM of the lowest energy emission. If done correctly, the virtual region created will contain a normalised sum of the previous regions, with an improved signal-to-noise ratio. Traditional peak analysis (or any other appropriate method) can then be performed to identify any net counts above the background. Note that whilst this is possible in both traditional and enhanced modes, extreme care must be taken in the former, where due to the multiplicity of the signal being captured (or lack thereof), it is more likely that an interference could falsely inflate the signal. Finally, particularly for traditional analysis, combined signals can be used to rule out false-positives. If a combined signal can improve the detection limit in the case where a detection is marginal, it should be used. A common example here is 60 Co; both the 1173.2 keV and 1332.5 keV emissions have an emission probability of almost 1, the differential in detector efficiency between the two energies is minimal, and the background in each region is similar. In most circumstances, therefore, combining the regions will create a greater signal-to-noise ratio.
4.1.3. Combining signatures In the previous calculation, two coincidence projections both contributed to the same signal (the 795.9 keV emission), and therefore could be easily combined to lower the detection limit below that achievable with a single coincidence signal. This was doubly-trivial as the background in each case was 0. The same principle, however, can be applied to any signal from the same nuclide. The resulting combined signal may improve or obscure a nuclide signal based upon the relative signal strengths and the background in each region. For example, using the traditional spectrum, the combination of the 604.7 keV and 795.9 keV regions for 134 Cs results in an MDC of ∼ 2.8×10−7 Bq m−3 , a ∼23% improvement over the 604.7 keV signature 3
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Fig. 4. Measurement results from both Traditional (left) and Enhanced (right) 𝛾-spectrometry.
It is clear from Table 1 that different radioisotopes benefit from different methodologies. Sensitivity for certain radioisotopes (144 Ce, 125 Sb) is maximised using traditional methods, whilst the majority are greatly improved by coincidence analysis; detection limits for many radioisotopes are improved by at least a factor of 10, and the detection limit of 136 Cs is improved by almost two orders of magnitude when using multi-signature enhanced analysis over the traditional method and a single detector. In >80% of the radionuclide MDC calculations, using multiple 𝛾 signatures improved the sensitivity achievable, with a maximum increase of 85% for 156 Eu when using enhanced analysis (at a signature multiplicity of 28). These results represent the limit of what is currently achievable with the research system at GBL15 (in this configuration). For the first time, detection sensitivities have been pushed into the nBq m−3 regime (equivalent to specific activities of ∼200 μBq.)
In this sample, the background regions contain 16915 and 15671 counts in the 1173.2 keV and 1332.5 keV regions. Combining these, the MDC reduces from ∼ 4.9 × 10−7 Bq m−3 to ∼ 3.6 × 10−7 Bq m−3 , an improvement of 27%. This equates to only needing ∼400 counts in each peak to provide a detectable signal, as opposed to the ∼600 required when analysing the peaks separately. 4.3. Evaluating the MDC Using MUGS, MDC calculations were performed as detailed in the previous sections. This includes values for traditional spectrometry, enhanced spectrometry, and optimised multi-signal spectrometry for both methodologies. These were initially performed without any restrictions on the multiplicity of the combined signatures, however it was quickly found that for enhanced spectrometry, the potential for large multiplicities is high. MUGS was therefore run with multiplicity limits in place, to evaluate the potential MDC gains as a function of multiplicity. This data is shown in Fig. 5. Fig. 5 clearly demonstrates the potential for combining multiple signatures, both for traditional and enhanced methodologies. Using traditional analysis, the maximum multiplicity with potential to improve the MDC is 5 (for 125 Sb), with the benefits tailing off past a multiplicity of 2. For enhanced analysis, it is possible to combine many more signals to achieve the ultimate MDC (up to 136 for 156 Eu), however the returns (percentage improvement in MDC above a single signature) quickly diminish. For this reason the maximum signature multiplicity for enhanced spectrometry was limited to 5 (and the traditional to 2). These optimised values were used for a final MUGS run, the results of which are included in Table 1. This table also compares the sensitivity achievable using traditional and enhanced methodologies.
5. Reaching the ultimate limit 5.1. Infinite detectors and whole signal spectroscopy Given the discussion thus far, it is natural to ask how much further the limits can be pushed. There are some fundamental constraints, although these are generally limited to the nuclide and the relevant decay mechanism. Alternative techniques, such as high-resolution 𝛽– 𝛾 spectrometry, will undoubtedly have a role to play in the future, allowing even more of the decay to be captured. For now though, we will limit ourselves to estimating the limits of the techniques discussed here (traditional and enhanced 𝛾 spectrometry). Firstly, let us assume that detectors of the future are infinite. Perhaps not in their physical extent, but big enough such that all radiation 4
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Fig. 5. Multiplicity calculations for the combination of signatures using MUGS. The percentage improvements refer to the MDC when compared to the lowest MDC possible with a single signature for each methodology. If there is no improvement possible past a certain signature multiplicity, no data is shown. Note that for enhanced spectrometry, it is possible to combine many more emissions and improve the MDC.
Table 1 MDC values for a selection of radionuclides and a range of techniques (note that the 140 La MDC is calculated assuming equilibrium with 140 Ba). All results are in nano Becquerels (nBq) per cubic metre of air sampled. Traditional 𝛾-spectrometry is split into single and dual detector systems. Each technique is further sub-divided into single signal and optimised multi-signal (MS). The improvement factor here refers to the ratio of the sensitivity using a standard, single detector 𝛾-spectrometer (the ‘Single’ column) to the technique with the greatest sensitivity for that nuclide. Nuclide
108𝑚 Ag 110𝑚
Ag Ba 140 Ba 144 Ce 60 Co 134 Cs 136 Cs 152 Eu 156 Eu 59 Fe 140 La 102 Rh 125 Sb 126 Sb 46 Sc 133
MDC (nBq m−3 ) Single
MS
Dual
MS
Coinc.
MS
Best technique
Improvement factor
329 412 3960 1010 639 489 358 487 2980 1040 726 605 558 708 434 390
272 360 2870 846 – 359 278 381 – – 597 446 – 674 329 306
233 291 2800 713 452 345 253 344 2100 738 513 427 394 500 307 275
192 254 2030 598 – 253 197 269 – – 422 315 – 476 233 216
7.6 19.9 501 195 21100 24.8 9.97 14.3 330 235 105 35.6 116 1620 19.3 15.7
4.8 8.47 494 – 12800 – 8.01 5.22 270 65.1 99.6 17 96.3 1100 11.5 –
Coinc. MS Coinc. MS Coinc. MS Coinc. Dual det. Coinc. Coinc. MS Coinc. MS Coinc. MS Coinc. MS Coinc. MS Coinc. MS Coinc. MS Dual det. MS Coinc. MS Coinc. MS
69 49 8 5.2 1.4 20 45 93 11 16 7.3 36 5.8 1.5 38 25
many orders of magnitude higher than what is possible using enhanced 𝛾-spectrometry.
in the sample will be fully captured by them. Next, we must consider the background. For enhanced spectrometry, the background has already been largely eliminated in certain scenarios, with the number of counts seen remaining at zero for multiple projections during a 30 day measurement. This was not true for all radionuclides, as Compton scattered 𝛾 emissions (which generate the Compton continuum) are also seen in the coincidence plane. Gating on 𝛾 emissions therefore often requires gating on a Compton continuum from a higher energy decay, and introducing background into the projected spectrum. It is worth noting that this effect is particularly acute when measuring samples of mixed fission products, which have several high-energy cascade emitters (such as 140 La). For traditional measurements, the background can be lowered substantially by the use of passive and active shielding, inert gases to flush any radon from the system, and deep underground laboratories. Once these background sources are eliminated, the largest contribution to the noise in the signal will again be Compton scattered 𝛾 emissions that originate from the sample. With the assumption of infinite detectors, it naturally follows that there will be no Compton continuum, as all 𝛾 emissions will be fully absorbed. It is worth noting that whilst background radiation rates can be extremely low for traditional methods, in general these are still
5.2. Evaluating the MDC To evaluate the detection limits possible using a 𝛾–𝛾 system as described in this work (albeit one with no losses and zero background), the efficiency functions that define the detectors performance (both peak and total) were artificially inflated to 50% across the entire energy range (the maximum geometric efficiency possible for a dual-detector system). Backgrounds were also reduced to zero, for both traditional and enhanced methods. RIMMER was then used to generate the quantification factors for all radionuclides in both traditional and enhanced modes, and MUGS again used to calculate the optimal multiplicity for combining these signatures. Fig. 6 summarises the MUGS results. The results for an idealised system naturally tend towards large multiplicities, as every additional signal results in a gain in sensitivity. To allow comparison with the current system, multiplicities were again limited to 2 & 5 for traditional and enhanced systems, respectively. The absolute results are detailed in Table 2. As previously demonstrated with both traditional and enhanced spectrometry, the more signal captured, the better the sensitivity achievable. In the idealised system, detection limits can approach 10– 100 pBq m−3 , with broadly similar sensitivities for both techniques 5
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Fig. 6. MUGS multiplicity calculations for infinite detectors and zero background. This represents the maximum possible improvement for each nuclide at each multiplicity. When all signals are combined, the ultimate MDC is achieved.
Table 2 MDC values for a selection of radionuclides (note that the 140 La MDC is calculated assuming equilibrium with 140 Ba). All results are in nano Becquerels (nBq) per cubic metre of air sampled. Both traditional and enhanced MDCs are presented for single and multiple (combined) signatures, which are recorded in the respective ‘MS’ columns and limited to a multiplicity of 2 & 5, respectively. The ratio of the current vs ‘ultimate’ sensitivity is also shown, with the current capability defined as the lowest MDC achieved in Table 1, and the ‘ultimate’ as the lowest MDC possible here. Nuclide
108𝑚
Ag 110𝑚 Ag 133 Ba 140 Ba 144 Ce 60 Co 134 Cs 136 Cs 152 Eu 156 Eu 59 Fe 140 La 102 Rh 125 Sb 126 Sb 46 Sc
MDC (nBq m−3 )
MS improvement (%)
Improvement ratio
Trad.
MS
Enhanced
MS
Trad.
Enhanced
Trad.
Enhanced
0.335 0.238 0.166 0.228 0.441 0.0978 0.114 0.187 0.291 0.505 0.0895 0.111 0.171 0.232 0.403 0.0979
0.167 0.134 0.0833 0.146 0.278 0.049 0.0598 0.106 0.165 0.276 0.0495 0.074 0.0854 0.116 0.201 0.049
199 41.4 0.509 1.59 95.9 0.0492 0.0707 0.37 0.384 0.943 1.63 0.149 0.411 0.553 10.2 0.0492
66.2 11.9 0.162 1.23 21.6 – 0.0539 0.21 0.246 0.412 1.13 0.0649 0.0833 0.146 7.59 –
50 44 50 36 37 50 48 43 43 45 45 33 50 50 50 50
67 71 68 23 77 – 24 43 36 56 30 57 80 74 25 –
814.4 1343.3 1248.5 3458.9 1147.5 3653.1 2324.4 2292.5 921.2 1898.6 6020.2 4094.6 3267.0 2913.8 815.9 3122.4
0.1 0.7 219.8 158.5 211.7 504.1 148.6 24.9 111.8 159.0 88.0 261.9 1152.5 7534.2 1.9 319.1
(when considering the combination of multiple signatures). This is expected, as we are now essentially considering how much signal you can capture, and the additional selectivity of enhanced 𝛾-spectrometry is not useful for reducing the background (as it is artificially set to zero).
5.3. How far is this from reality? A zero background regime is practically achievable with enhanced 𝛾-spectrometry, with the background largely eliminated by the selectivity of the cascade, and the only remaining noise due to Compton scattered 𝛾 emissions. This could be substantially reduced using active Compton suppression shields, however care must be taken not to expose these shields to the source directly. For traditional measurements, the background can be substantially reduced but not eliminated. Experiments in deep underground facilities suggest that 1–2 orders of magnitude reduction may be possible, and Compton suppression shields could again be used to minimise noise generated by incomplete 𝛾 deposition in the detectors. For this system (with a detector multiplicity of two) cascade summing reduces the useful signal by up to an order of magnitude. This could be mitigated by using a larger number of detectors, further away from the source. Such systems are often utilised in fundamental research and nuclear structure studies [12,13] , however each experimental campaign can require years of analysis, and the system complexity (and cost) are beyond what is currently possible in an environmental laboratory. A note on materials — whilst traditional systems are likely to continue to rely on HPGe detectors (and the resolution they afford), the ultimate system (when capturing several 𝛾 emissions from a 𝛾
For almost all radionuclides tested here, is clear that order of magnitude improvements over the current limits are still possible. Enhanced spectrometry is closer to the ultimate limit possible, with an average potential improvement ratio of ∼681 compared to ∼2459 for traditional spectrometry. It is worth noting that for enhanced spectrometry, the standard deviation of this factor is almost 50% larger that of traditional spectrometry, highlighting the natural variability in the potential of enhanced spectrometry, and its sensitivity to the decay mechanism of each isotope. Table 2 also highlights one other interesting scenario, where enhanced spectrometry actually performs worse with infinite detectors (an improvement ratio <1). For 108𝑚 Ag, almost all coincidence signatures come from the same 3-𝛾 decay chain, which cannot be resolved by a two-detector system. The inefficiency of the current (realistic) system actually aids detection, with the third 𝛾 sometimes able to escape. If the number of detectors were increased, the detection limit would drop considerably for such signatures. 6
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cascade) could sacrifice resolution for practicality/efficiency. LaBr3 (Ce) detectors are promising in this regard, as they are cheaper than HPGe, can be operated at room temperature, and exhibit much faster decay times (dramatically increasing selectivity, and therefore reducing background). These would only be of use, however, for coincidence spectroscopy, and not for extracting single emitters (due to the inherent background in the detector). Emerging technologies, such as Transition Edge Sensors (TES) [14], are also rapidly advancing, demonstrating laboratory scale sensors with an energy resolution of 2.1 eV at 5.9 keV [15]. The cooling requirements currently limit the practicality of these systems, however they remain an interesting proposition for increasing the selectivity of a detector system.
This system has achieved some of the lowest detection limits seen in the literature when operating in coincidence mode, increasing sensitivity by up to a factor of 100. The ultimate limit for this technique has been calculated, highlighting the potential for a further 10–1000 fold improvement. Whilst the assumptions required for this calculation are beyond detector technology for the foreseeable future, there are several practical steps that can be taken to improve detection sensitivity further. These include the addition of larger HPGe crystals, Compton suppression shields, additional detector technologies to capture more of the nuclide decay (such as 𝛽-cells and silicon strip detectors), and the deployment of these systems deep underground. All of these avenues are being actively explored in an attempt to develop the ultimate omni-spectrometer.
6. Discussion & conclusions Acknowledgement Using a low-background dual-detector system, detection limits have been improved from ∼10 μBq m−3 to ∼1 μBq m−3 for a week long measurement of 140 Ba using single cascade coincidence analysis. For other nuclides, sensitivity improvements have been demonstrated up to a factor of 50 using coincidence spectrometry, and detection levels have moved from the μBq to the nBq regime. Much longer counts than a week are possible using the dedicated facility at GBL15, and a successor system is planned for deployment into Boubly Underground Laboratory, greatly reducing the background seen when analysing in traditional modes. The average improvement (excluding 144 Ce and 125 Sb) obtained using single cascade coincidence spectrometry is a factor of ∼5 greater than what is possible with a dual-detector system operating in traditional mode. By including multiple cascades, the sensitivity can be further improved by an average of 35%, realising a total improvement factor of ∼7. This clearly demonstrates the benefits of fully utilising a dual-detector system by exploiting coincidence analysis. MDC analysis and signal quantification in these modes is complex, but possible due to the use of RIMMER. This has been automated in GBL15 via the use of MUGS, which uses a brute force method to identify the best signatures to use/combine for detection or sensitivity analysis. To combine signals in discrete regions, the concept of a virtual peak is introduced, allowing improved signal-to-noise ratios across a vast range of nuclides in all analysis modes. This will particularly benefit applications where false-positives are an issue.
The authors wish to thank Kurt Ungar of Health Canada, who provided samples for enhanced analysis. References [1] R. Britton, J. Burnett, A. Davies, M. Jackson, J. Environ. Radioact. 146 (2015) 1–5. [2] R. Britton, M. Jackson, A. Davies, J. Environ. Radioact. 149 (2015) 158–163. [3] R. Britton, M. Jackson, A. Davies, J. Appl. Radiat. Isot. 116 (2016) 128–133. [4] M. Jackson, R. Britton, A. Davies, J. McLarty, M. Goodwin, Nucl. Instrum. Methods Phys. Res. A 834 (2016) 158–163. [5] J. Burnett, R. Britton, D. Abrecht, A. Davies, J. Radioanal. Nucl. Chem. 313 (2017) 191–196. [6] R. Britton, A. Davies, Nucl. Instrum. Methods Phys. Res. A 940 (2019) 215–222. [7] R. Britton, A. Davies, Nucl. Instrum. Methods Phys. Res. A 786 (2015) 12–16. [8] J. Burnett, A. Davies, J. McLarty, J. Radioanal. Nucl. Chem. 298–2 (2013) 987–992. [9] J. Burnett, Nucl. Instrum. Methods Phys. Res. A 944 (2019) 162473. [10] R. Brun, F. Rademakers, Nucl. Instrum. Methods Phys. Res. A 389 (1997) 81–86. [11] M. Bhat, Evaluated nuclear structure data file (ENSDF), Nucl. Data Sci. Technol. 81 (1992) 7–821. [12] W. Gelletly, J. Eberth, Lect. Notes Phys. 700 (2006) 79–117. [13] M. Riley, J. Simpson, Nuclear 𝛾-spectroscopy and the 𝛾-spheres, in: KGaA (Ed.), Digital Encyclopedia of Applied Physics, Wiley-VCH Verlag GmbH & Co, 2014. [14] J.N. Ullom, D.A. Bennett, Supercond. Sci. Technol. 28 (8) (2015) 084003. [15] W.B. Doriese, et al., Rev. Sci. Instrum. 88 (5) (2017) 053108.
7
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Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Limits of detection — Enhancing identification of anthropogenic radionuclides R. Britton ∗, A.V. Davies AWE, Aldermaston, Reading, Berkshire, RG7 4PR, UK
ARTICLE
INFO
Keywords: Coincidence spectrometry Gamma spectroscopy List-mode Detection limits CTBT
ABSTRACT The sensitivity of 𝛾-spectrometry is defined by three fundamental factors — the particulars of each isotopes decay mechanism, the losses in the detector system, and the background radiation present during the measurement. This ‘glass floor’ therefore differs for each nuclide, but has the latter two components in common. Conventional measurements aim to minimise these common components, and therefore improve sensitivity for all isotopes. This research discusses the current (operational) sensitivity levels for 𝛾-spectrometry, and how new systems can improve upon these by a factor of 103 –106 . Finally, the ultimate sensitivity achievable is considered, along with practical considerations and potential improvements to get closer to this limit.
1. Introduction Gamma (𝛾) spectrometry is used in a diverse range of fields, from food safety studies to the verification of nuclear material. The limit of its application lies in two key considerations — the presence of a radionuclide that emits 𝛾 radiation in the sample to be measured, and the sensitivity of the detection systems available to laboratories. At GBL15 (the UK Comprehensive Nuclear-Test-Ban Treaty measurement facility based at AWE, Aldermaston), traditional 𝛾-spectrometry has been in use for decades. GBL15 now supports a number of international non-proliferation and treaty verification efforts. As part of these efforts, the laboratory has been focusing on developing revolutionary technologies that can provide a sea-change in the sensitivity achievable. Substantial research efforts have been invested in coincidence based systems [1,2], deep-underground systems, and the operational analysis of complex datasets [3–6]. During these efforts, it has become clear that 𝛾-spectrometry has not yet approached its ultimate limit, and pushing the current limits may yield a number of new applications for what is already an incredibly useful non-destructive analytical method; this paper considers both the advances made to date and the potential gains to be made in the future. 2. Traditional 𝜸-spectrometry High-resolution 𝛾-spectrometry relies heavily on High Purity Germanium (HPGe) based detectors, as no other semi-conductor or scintillation based device can match the performance (due to the combination of efficiency and resolution) achievable with these systems. Samples often contain hundreds of peaks, which would be unresolvable with
Fig. 1. A typical detector system undergoing testing prior to deployment at RN67, the CTBT International Monitoring System (IMS) station on St Helena. A single crystal detector is shielded with copper, tin, and lead. The entire shield is enclosed in BC408 plastic scintillation plates to veto cosmic induced events in the HPGe detector.
any scintillation based device (such as NaI(Tl)). Similarly, other semiconductor based devices, such as silicon detectors, cannot yet be made with large enough crystals to have a substantial efficiency above a few hundred keV. In the field of HPGe based 𝛾-spectrometry, crystals have become bigger, and recent advances in the electrical contacts have allowed greater resolution for planar crystals and well detectors (such as the Small Anode Germanium – SAGe – well designs from Mirion [7]). Electronics have also improved, with fully digitised Multi-Channel Analysers (MCAs) now the norm. Fundamentally, however, a 𝛾-spectrometer
∗ Corresponding author. E-mail address: [email protected] (R. Britton).
https://doi.org/10.1016/j.nima.2019.162818 Received 16 August 2019; Received in revised form 17 September 2019; Accepted 18 September 2019 Available online 21 September 2019 0168-9002/Crown Copyright © 2019 Published by Elsevier B.V. All rights reserved.
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looks the same as 20 years previous — a single detector HPGe crystal surrounded with lead to reduce the terrestrial background, possibly with a plastic scintillation plate to veto cosmic radiation contributions to the background [8], see Fig. 1. In a surface based laboratory (such as GBL15), the HPGe detectors used are typically the largest that are commercially available, and built using materials specifically selected for a low radiation background. Background is further reduced using a shield of 100 mm of lead (containing a small percentage of antimony), and another 50 mm of aged lead inside of this. Liners of tin, cadmium, and copper are often used to attenuate X-rays, and a BC408 plastic scintillation plate (typically 1 m2 ) is used to suppress cosmic radiation induced events in the crystal [9]. With all of these techniques, sensitivity is limited to around the 10 μBq/m−3 level for 140 Ba in a high-volume air filter that is counted for a week. Given a typical air collection volume of ∼20000 m3 in 24 h, this equates to a specific activity of ∼0.2 Bq in the sample. During the analysis, single peaks are used for the quantification of radionuclides (typically those with the highest emission probability) with other peaks used if present to calculate a weighted mean for the activity level. It is worth noting that the losses in the signal collection/analysis chain are around a factor of 100, based on a 10% branching ratio, and (a somewhat optimistic) 10% peak efficiency.
Fig. 2. The research system at GBL15, with a large internal cavity to minimise backscatter of 𝛾’s from the lead into the detectors. It is built in a modular fashion, and as such can include a number of NaI detectors to act as a Compton shield, and silicon pin based 𝛽 cells. The inset shows the power supplies for the coolers, a LYNX™ MCA per detector, and the HEXAGON™ MCA.
3. Enhanced 𝜸-spectrometry 3.1. The detector system One way to improve the detection sensitivity is to increase the efficiency. Well type detectors are excellent in this regard, however suffer acutely from cascade summing effects [7]. This is where a cascade of 𝛾 radiation (which is fairly typical of how most nuclei decay) is captured in the detector, creating a signal with an amplitude that is somewhere between 0 keV and the total energy of the cascade. Given the variability in this signal, it cannot currently be reliably used for nuclide identification/quantification. Given that the overriding aim of all research systems at GBL15 is to improve operational capabilities, simplicity is key. The simplest solution that circumvents this issue is to include a second detector, as this doubles the detection efficiency and does not increase the systems sensitivity to cascade summing. Crucially, it also allows the detection and identification of 𝛾 cascades, which can provide an often unique signature that is far more distinctive than the single peaks used in traditional 𝛾 spectrometry. The research system at GBL15 consists of two electrically cooled BEGe 6530 detectors from Mirion. Multiple data acquisition chains are used, including LYNX™ MCAs from Mirion and a CAEN S.p.A HEXAGON™ dual-channel MCA. The detectors are situated in a faceto-face configuration, and enclosed in 150 mm lead with a retractable lid mounted on a rack-and-pinion system. Four 1 m2 BC408 plastic scintillation plates sit atop of the lid, providing suppression of cosmic events. All data is collected in time-stamped list-mode, with channels in each electronics chain time-synchronised (the clock bases differ for the two electronics chains — the LYNX™ units have a 100 MHz clock, whereas the HEXAGON™ has a 1 GHz clock). Up to 6 NaI detectors can also be inserted to provide Compton suppression during both traditional and enhanced operation, however none were in place for the analyses presented within this work (see Fig. 2).
Fig. 3. A traditional spectrum (blue) collected from a fission product sample measured on a HPGe detector. The total spectrum from all coincidences (𝛾 cascades) is shown in red. By requiring that one detector measures the 1596 keV 𝛾 from 140 La, the gated coincidence spectrum (green) results in an extremely clean nuclide signature, with almost all identified peaks arising from 140 La. All other nuclides (which are effectively background when characterising the 140 La signature) are dramatically reduced by a factor of 103 –104 . Note that this data can be sliced multiple times, creating clean coincidence projections for each nuclide of interest. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
loss in coincidence mode is therefore from the extra efficiency term of the second detector (assuming this also is around 10%, the total loss in the analysis chain is now around 1000). Whilst this is an order of magnitude more than in a traditional system, the selectivity available allows the background to be reduced from thousands of counts to just a few in the region of interest, and for very clean nuclide signatures to be obtained (see Fig. 3). To extract these signatures, GBL15 have developed C++/ROOT [10] based software known as the ‘AnalysisPackage’. This sorts all list-mode data into ROOT trees, extracts coincidences based upon variable timing windows, and builds both the histograms and matrices required for efficient analysis. The AnalysisPackage can gain-match and calibrate these data structures, and also includes the functionality to create advanced energy and time projections based upon multiple gates, fitting peaks/decay curves to the resulting projections as appropriate. To calculate the factors required to quantify these signatures, GBL15 use RIMMER [2,3], which only requires accurate peak/total efficiency characterisations and the appropriate ENSDF data [11] to calculate quantification factors for any single detector or coincidence based system.
3.2. Analysis Analysis of data from such a system is more complex than with a traditional system, however not prohibitively so. Data can simply be summed to create a detector with twice the efficiency, or events timecorrelated to analyse the cascades of 𝛾 radiation emitted. Whilst these cascades are isotope dependent, many have similar probabilities to the single emissions (within an order of magnitude). The only additional
4. Maximising detection sensitivity To quantify the detection limits possible with the research system, a 30 day acquisition was undertaken with a real sample. This 2
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on its own. Adding a third region (the 569.3 keV) reduces the sensitivity to ∼ 3.2 × 10−7 Bq m−3 due to the lower detectability of this emission when compared to the other 𝛾 signatures (the background in all three regions is similar). Identifying which combinations are optimal is inherently complex, as this cannot be calculated without the final histogram/matrix (which are used to determine the background, either directly from the gainmatched sum spectrum or from the sum of the projected coincidence spectra). Using the output of RIMMER (which contains the top twenty detectable signatures for both traditional and enhanced analysis for all radionuclides of interest) in combination with the ROOT file created by the AnalysisPackage, a ‘brute-force’ approach is used to evaluate all possible combinations of signatures and find the optimal subset. This process has been highly automated using a small PyROOT based application, known in GBL15 as MUGS (MUlti-Gamma Surveyor). This imports the RIMMER data, and uses it to identify regions and calculate background integrals for every emission in the RIMMER library. MUGS then moves onto the coincidence matrix, creating projections for each 𝛾-energy pair and evaluating the resulting integral in the region of interest. Finally, MUGS calculates the MDC for each signature on its own, followed by every possible combination of signatures that could be used for each nuclide. The analyst is then presented with the top three signatures (along with the MDC) for traditional, enhanced, and multi-signature analysis. It is worth noting that in practice, it is unlikely for many emissions to combine and provide a vastly more sensitive result than a single signature alone. Typically, one or two dominant signatures will provide the best result, and this is (of course) dependent on the sample being measured. The entire process (including the processing of 30 days of listmode data using the AnalysisPackage, running RIMMER to generate the quantification factors and 𝛾-cascade detection probabilities, and then using MUGS to calculate and identify the optimal signatures for each methodology and nuclide) takes less than a minute.
was from eastern Europe, and was one sample in a series of collections that detected 106 Rh during September and October 2017. Fig. 4 shows the measurement results using both traditional and enhanced methodologies. In Fig. 4, clear signals are seen from 106 Rh, which is a daughter of the 106 Ru that was collected on the air filter. To calculate the sensitivity levels for other radionuclides, the level of background can be used in each region of interest to calculate a critical limit, either based on the Currie method (where the number of counts in the region is great enough for a Gaussian approximation to be valid) or directly using Poisson statistics. The regions are automatically determined using RIMMER, which (in this study) is configured to provide a list of the top 20 signatures for each methodology and nuclide with the highest detectability. The sample was measured in March/April 2018, and all sensitivity levels are decay corrected to the start of acquisition. 4.1. An example with
134 Cs
4.1.1. Traditional 𝛾-spectrometry Using a single detector, a peak efficiency of 4.3% for the 604.72 keV emission, an abundance of 97.6% and a cascade summing correction of 0.806 (both calculated using RIMMER), the total loss factor is ∼29; i.e. the number of counts in this peak (if 134 Cs was present in the sample) would be 29 times lower than the number of decays that occurred. For both detectors, the loss factor is obviously half of this. There are 25776 background counts in this peak region – defined as the FWHM (Full-Width at Half-Maximum) of the detector at this energy multiplied by 1.25 – which results in a critical limit of 749. This is the number of counts required for statistical significance, and when combined with the loss factor, requires the total number of decays from 134 Cs in the sample to exceed 20851 during the measurement for confirmation of the detection to occur. Given that the total count time was 2431920 s, and the collection volume 25194 m3 , the Minimum Detectable Concentration (MDC) is ∼ 3.6 × 10−7 Bq m−3 (or ∼ 2.5 × 10−7 Bq m−3 for both detectors).
4.2. Virtual peaks and false-positives
4.1.2. Enhanced 𝛾-spectrometry Using the coincidence detection capabilities of the full system, a number of emissions can be included to contribute to the signal; the 795.9 keV emission is detected in coincidence with both the 604.7 keV emission 0.246% of the time, and the 569.3 keV emission 0.0326% of the time (calculated using RIMMER). Using the AnalysisPackage to gate on both of these signals, the total 795.9 keV peak can be projected, creating a signal with a total loss factor of ∼296 (note that separately, the loss factors would be ∼406 and ∼3067 respectively). Despite the combined loss factor being ∼1 order of magnitude higher than the traditional measurement, the background in this region, as a result of both projections, is 0. This equates to a critical limit of 3, and therefore the total number of decays required for detection to exceed 1078 during the measurement. The MDC is therefore ∼ 1.8 × 10−8 Bq m−3 , ∼10% lower than when using a single coincidence pair, and a factor of ∼20 lower than when using the most effective single emission (604.7 keV).
When going beyond a mere sensitivity study (and attempting to detect a nuclide) one must consider how to combine these signatures in the analysis. For coincidence pairs with a common energy, this is trivial, as gating on the non-common energy will result in perfectly aligned projections. When the energies are different, the analyst must consider the idea of a ‘Virtual Peak’. Here, the analysis regions identified using MUGS must be carefully normalised and re-binned (preferably to the lowest energy peak) so that they can be recombined. This recombination must account for both the potential variability in the binning of the original signal, and the compressing of the region to the reduced FWHM of the lowest energy emission. If done correctly, the virtual region created will contain a normalised sum of the previous regions, with an improved signal-to-noise ratio. Traditional peak analysis (or any other appropriate method) can then be performed to identify any net counts above the background. Note that whilst this is possible in both traditional and enhanced modes, extreme care must be taken in the former, where due to the multiplicity of the signal being captured (or lack thereof), it is more likely that an interference could falsely inflate the signal. Finally, particularly for traditional analysis, combined signals can be used to rule out false-positives. If a combined signal can improve the detection limit in the case where a detection is marginal, it should be used. A common example here is 60 Co; both the 1173.2 keV and 1332.5 keV emissions have an emission probability of almost 1, the differential in detector efficiency between the two energies is minimal, and the background in each region is similar. In most circumstances, therefore, combining the regions will create a greater signal-to-noise ratio.
4.1.3. Combining signatures In the previous calculation, two coincidence projections both contributed to the same signal (the 795.9 keV emission), and therefore could be easily combined to lower the detection limit below that achievable with a single coincidence signal. This was doubly-trivial as the background in each case was 0. The same principle, however, can be applied to any signal from the same nuclide. The resulting combined signal may improve or obscure a nuclide signal based upon the relative signal strengths and the background in each region. For example, using the traditional spectrum, the combination of the 604.7 keV and 795.9 keV regions for 134 Cs results in an MDC of ∼ 2.8×10−7 Bq m−3 , a ∼23% improvement over the 604.7 keV signature 3
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Fig. 4. Measurement results from both Traditional (left) and Enhanced (right) 𝛾-spectrometry.
It is clear from Table 1 that different radioisotopes benefit from different methodologies. Sensitivity for certain radioisotopes (144 Ce, 125 Sb) is maximised using traditional methods, whilst the majority are greatly improved by coincidence analysis; detection limits for many radioisotopes are improved by at least a factor of 10, and the detection limit of 136 Cs is improved by almost two orders of magnitude when using multi-signature enhanced analysis over the traditional method and a single detector. In >80% of the radionuclide MDC calculations, using multiple 𝛾 signatures improved the sensitivity achievable, with a maximum increase of 85% for 156 Eu when using enhanced analysis (at a signature multiplicity of 28). These results represent the limit of what is currently achievable with the research system at GBL15 (in this configuration). For the first time, detection sensitivities have been pushed into the nBq m−3 regime (equivalent to specific activities of ∼200 μBq.)
In this sample, the background regions contain 16915 and 15671 counts in the 1173.2 keV and 1332.5 keV regions. Combining these, the MDC reduces from ∼ 4.9 × 10−7 Bq m−3 to ∼ 3.6 × 10−7 Bq m−3 , an improvement of 27%. This equates to only needing ∼400 counts in each peak to provide a detectable signal, as opposed to the ∼600 required when analysing the peaks separately. 4.3. Evaluating the MDC Using MUGS, MDC calculations were performed as detailed in the previous sections. This includes values for traditional spectrometry, enhanced spectrometry, and optimised multi-signal spectrometry for both methodologies. These were initially performed without any restrictions on the multiplicity of the combined signatures, however it was quickly found that for enhanced spectrometry, the potential for large multiplicities is high. MUGS was therefore run with multiplicity limits in place, to evaluate the potential MDC gains as a function of multiplicity. This data is shown in Fig. 5. Fig. 5 clearly demonstrates the potential for combining multiple signatures, both for traditional and enhanced methodologies. Using traditional analysis, the maximum multiplicity with potential to improve the MDC is 5 (for 125 Sb), with the benefits tailing off past a multiplicity of 2. For enhanced analysis, it is possible to combine many more signals to achieve the ultimate MDC (up to 136 for 156 Eu), however the returns (percentage improvement in MDC above a single signature) quickly diminish. For this reason the maximum signature multiplicity for enhanced spectrometry was limited to 5 (and the traditional to 2). These optimised values were used for a final MUGS run, the results of which are included in Table 1. This table also compares the sensitivity achievable using traditional and enhanced methodologies.
5. Reaching the ultimate limit 5.1. Infinite detectors and whole signal spectroscopy Given the discussion thus far, it is natural to ask how much further the limits can be pushed. There are some fundamental constraints, although these are generally limited to the nuclide and the relevant decay mechanism. Alternative techniques, such as high-resolution 𝛽– 𝛾 spectrometry, will undoubtedly have a role to play in the future, allowing even more of the decay to be captured. For now though, we will limit ourselves to estimating the limits of the techniques discussed here (traditional and enhanced 𝛾 spectrometry). Firstly, let us assume that detectors of the future are infinite. Perhaps not in their physical extent, but big enough such that all radiation 4
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Fig. 5. Multiplicity calculations for the combination of signatures using MUGS. The percentage improvements refer to the MDC when compared to the lowest MDC possible with a single signature for each methodology. If there is no improvement possible past a certain signature multiplicity, no data is shown. Note that for enhanced spectrometry, it is possible to combine many more emissions and improve the MDC.
Table 1 MDC values for a selection of radionuclides and a range of techniques (note that the 140 La MDC is calculated assuming equilibrium with 140 Ba). All results are in nano Becquerels (nBq) per cubic metre of air sampled. Traditional 𝛾-spectrometry is split into single and dual detector systems. Each technique is further sub-divided into single signal and optimised multi-signal (MS). The improvement factor here refers to the ratio of the sensitivity using a standard, single detector 𝛾-spectrometer (the ‘Single’ column) to the technique with the greatest sensitivity for that nuclide. Nuclide
108𝑚 Ag 110𝑚
Ag Ba 140 Ba 144 Ce 60 Co 134 Cs 136 Cs 152 Eu 156 Eu 59 Fe 140 La 102 Rh 125 Sb 126 Sb 46 Sc 133
MDC (nBq m−3 ) Single
MS
Dual
MS
Coinc.
MS
Best technique
Improvement factor
329 412 3960 1010 639 489 358 487 2980 1040 726 605 558 708 434 390
272 360 2870 846 – 359 278 381 – – 597 446 – 674 329 306
233 291 2800 713 452 345 253 344 2100 738 513 427 394 500 307 275
192 254 2030 598 – 253 197 269 – – 422 315 – 476 233 216
7.6 19.9 501 195 21100 24.8 9.97 14.3 330 235 105 35.6 116 1620 19.3 15.7
4.8 8.47 494 – 12800 – 8.01 5.22 270 65.1 99.6 17 96.3 1100 11.5 –
Coinc. MS Coinc. MS Coinc. MS Coinc. Dual det. Coinc. Coinc. MS Coinc. MS Coinc. MS Coinc. MS Coinc. MS Coinc. MS Coinc. MS Dual det. MS Coinc. MS Coinc. MS
69 49 8 5.2 1.4 20 45 93 11 16 7.3 36 5.8 1.5 38 25
many orders of magnitude higher than what is possible using enhanced 𝛾-spectrometry.
in the sample will be fully captured by them. Next, we must consider the background. For enhanced spectrometry, the background has already been largely eliminated in certain scenarios, with the number of counts seen remaining at zero for multiple projections during a 30 day measurement. This was not true for all radionuclides, as Compton scattered 𝛾 emissions (which generate the Compton continuum) are also seen in the coincidence plane. Gating on 𝛾 emissions therefore often requires gating on a Compton continuum from a higher energy decay, and introducing background into the projected spectrum. It is worth noting that this effect is particularly acute when measuring samples of mixed fission products, which have several high-energy cascade emitters (such as 140 La). For traditional measurements, the background can be lowered substantially by the use of passive and active shielding, inert gases to flush any radon from the system, and deep underground laboratories. Once these background sources are eliminated, the largest contribution to the noise in the signal will again be Compton scattered 𝛾 emissions that originate from the sample. With the assumption of infinite detectors, it naturally follows that there will be no Compton continuum, as all 𝛾 emissions will be fully absorbed. It is worth noting that whilst background radiation rates can be extremely low for traditional methods, in general these are still
5.2. Evaluating the MDC To evaluate the detection limits possible using a 𝛾–𝛾 system as described in this work (albeit one with no losses and zero background), the efficiency functions that define the detectors performance (both peak and total) were artificially inflated to 50% across the entire energy range (the maximum geometric efficiency possible for a dual-detector system). Backgrounds were also reduced to zero, for both traditional and enhanced methods. RIMMER was then used to generate the quantification factors for all radionuclides in both traditional and enhanced modes, and MUGS again used to calculate the optimal multiplicity for combining these signatures. Fig. 6 summarises the MUGS results. The results for an idealised system naturally tend towards large multiplicities, as every additional signal results in a gain in sensitivity. To allow comparison with the current system, multiplicities were again limited to 2 & 5 for traditional and enhanced systems, respectively. The absolute results are detailed in Table 2. As previously demonstrated with both traditional and enhanced spectrometry, the more signal captured, the better the sensitivity achievable. In the idealised system, detection limits can approach 10– 100 pBq m−3 , with broadly similar sensitivities for both techniques 5
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Fig. 6. MUGS multiplicity calculations for infinite detectors and zero background. This represents the maximum possible improvement for each nuclide at each multiplicity. When all signals are combined, the ultimate MDC is achieved.
Table 2 MDC values for a selection of radionuclides (note that the 140 La MDC is calculated assuming equilibrium with 140 Ba). All results are in nano Becquerels (nBq) per cubic metre of air sampled. Both traditional and enhanced MDCs are presented for single and multiple (combined) signatures, which are recorded in the respective ‘MS’ columns and limited to a multiplicity of 2 & 5, respectively. The ratio of the current vs ‘ultimate’ sensitivity is also shown, with the current capability defined as the lowest MDC achieved in Table 1, and the ‘ultimate’ as the lowest MDC possible here. Nuclide
108𝑚
Ag 110𝑚 Ag 133 Ba 140 Ba 144 Ce 60 Co 134 Cs 136 Cs 152 Eu 156 Eu 59 Fe 140 La 102 Rh 125 Sb 126 Sb 46 Sc
MDC (nBq m−3 )
MS improvement (%)
Improvement ratio
Trad.
MS
Enhanced
MS
Trad.
Enhanced
Trad.
Enhanced
0.335 0.238 0.166 0.228 0.441 0.0978 0.114 0.187 0.291 0.505 0.0895 0.111 0.171 0.232 0.403 0.0979
0.167 0.134 0.0833 0.146 0.278 0.049 0.0598 0.106 0.165 0.276 0.0495 0.074 0.0854 0.116 0.201 0.049
199 41.4 0.509 1.59 95.9 0.0492 0.0707 0.37 0.384 0.943 1.63 0.149 0.411 0.553 10.2 0.0492
66.2 11.9 0.162 1.23 21.6 – 0.0539 0.21 0.246 0.412 1.13 0.0649 0.0833 0.146 7.59 –
50 44 50 36 37 50 48 43 43 45 45 33 50 50 50 50
67 71 68 23 77 – 24 43 36 56 30 57 80 74 25 –
814.4 1343.3 1248.5 3458.9 1147.5 3653.1 2324.4 2292.5 921.2 1898.6 6020.2 4094.6 3267.0 2913.8 815.9 3122.4
0.1 0.7 219.8 158.5 211.7 504.1 148.6 24.9 111.8 159.0 88.0 261.9 1152.5 7534.2 1.9 319.1
(when considering the combination of multiple signatures). This is expected, as we are now essentially considering how much signal you can capture, and the additional selectivity of enhanced 𝛾-spectrometry is not useful for reducing the background (as it is artificially set to zero).
5.3. How far is this from reality? A zero background regime is practically achievable with enhanced 𝛾-spectrometry, with the background largely eliminated by the selectivity of the cascade, and the only remaining noise due to Compton scattered 𝛾 emissions. This could be substantially reduced using active Compton suppression shields, however care must be taken not to expose these shields to the source directly. For traditional measurements, the background can be substantially reduced but not eliminated. Experiments in deep underground facilities suggest that 1–2 orders of magnitude reduction may be possible, and Compton suppression shields could again be used to minimise noise generated by incomplete 𝛾 deposition in the detectors. For this system (with a detector multiplicity of two) cascade summing reduces the useful signal by up to an order of magnitude. This could be mitigated by using a larger number of detectors, further away from the source. Such systems are often utilised in fundamental research and nuclear structure studies [12,13] , however each experimental campaign can require years of analysis, and the system complexity (and cost) are beyond what is currently possible in an environmental laboratory. A note on materials — whilst traditional systems are likely to continue to rely on HPGe detectors (and the resolution they afford), the ultimate system (when capturing several 𝛾 emissions from a 𝛾
For almost all radionuclides tested here, is clear that order of magnitude improvements over the current limits are still possible. Enhanced spectrometry is closer to the ultimate limit possible, with an average potential improvement ratio of ∼681 compared to ∼2459 for traditional spectrometry. It is worth noting that for enhanced spectrometry, the standard deviation of this factor is almost 50% larger that of traditional spectrometry, highlighting the natural variability in the potential of enhanced spectrometry, and its sensitivity to the decay mechanism of each isotope. Table 2 also highlights one other interesting scenario, where enhanced spectrometry actually performs worse with infinite detectors (an improvement ratio <1). For 108𝑚 Ag, almost all coincidence signatures come from the same 3-𝛾 decay chain, which cannot be resolved by a two-detector system. The inefficiency of the current (realistic) system actually aids detection, with the third 𝛾 sometimes able to escape. If the number of detectors were increased, the detection limit would drop considerably for such signatures. 6
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cascade) could sacrifice resolution for practicality/efficiency. LaBr3 (Ce) detectors are promising in this regard, as they are cheaper than HPGe, can be operated at room temperature, and exhibit much faster decay times (dramatically increasing selectivity, and therefore reducing background). These would only be of use, however, for coincidence spectroscopy, and not for extracting single emitters (due to the inherent background in the detector). Emerging technologies, such as Transition Edge Sensors (TES) [14], are also rapidly advancing, demonstrating laboratory scale sensors with an energy resolution of 2.1 eV at 5.9 keV [15]. The cooling requirements currently limit the practicality of these systems, however they remain an interesting proposition for increasing the selectivity of a detector system.
This system has achieved some of the lowest detection limits seen in the literature when operating in coincidence mode, increasing sensitivity by up to a factor of 100. The ultimate limit for this technique has been calculated, highlighting the potential for a further 10–1000 fold improvement. Whilst the assumptions required for this calculation are beyond detector technology for the foreseeable future, there are several practical steps that can be taken to improve detection sensitivity further. These include the addition of larger HPGe crystals, Compton suppression shields, additional detector technologies to capture more of the nuclide decay (such as 𝛽-cells and silicon strip detectors), and the deployment of these systems deep underground. All of these avenues are being actively explored in an attempt to develop the ultimate omni-spectrometer.
6. Discussion & conclusions Acknowledgement Using a low-background dual-detector system, detection limits have been improved from ∼10 μBq m−3 to ∼1 μBq m−3 for a week long measurement of 140 Ba using single cascade coincidence analysis. For other nuclides, sensitivity improvements have been demonstrated up to a factor of 50 using coincidence spectrometry, and detection levels have moved from the μBq to the nBq regime. Much longer counts than a week are possible using the dedicated facility at GBL15, and a successor system is planned for deployment into Boubly Underground Laboratory, greatly reducing the background seen when analysing in traditional modes. The average improvement (excluding 144 Ce and 125 Sb) obtained using single cascade coincidence spectrometry is a factor of ∼5 greater than what is possible with a dual-detector system operating in traditional mode. By including multiple cascades, the sensitivity can be further improved by an average of 35%, realising a total improvement factor of ∼7. This clearly demonstrates the benefits of fully utilising a dual-detector system by exploiting coincidence analysis. MDC analysis and signal quantification in these modes is complex, but possible due to the use of RIMMER. This has been automated in GBL15 via the use of MUGS, which uses a brute force method to identify the best signatures to use/combine for detection or sensitivity analysis. To combine signals in discrete regions, the concept of a virtual peak is introduced, allowing improved signal-to-noise ratios across a vast range of nuclides in all analysis modes. This will particularly benefit applications where false-positives are an issue.
The authors wish to thank Kurt Ungar of Health Canada, who provided samples for enhanced analysis. References [1] R. Britton, J. Burnett, A. Davies, M. Jackson, J. Environ. Radioact. 146 (2015) 1–5. [2] R. Britton, M. Jackson, A. Davies, J. Environ. Radioact. 149 (2015) 158–163. [3] R. Britton, M. Jackson, A. Davies, J. Appl. Radiat. Isot. 116 (2016) 128–133. [4] M. Jackson, R. Britton, A. Davies, J. McLarty, M. Goodwin, Nucl. Instrum. Methods Phys. Res. A 834 (2016) 158–163. [5] J. Burnett, R. Britton, D. Abrecht, A. Davies, J. Radioanal. Nucl. Chem. 313 (2017) 191–196. [6] R. Britton, A. Davies, Nucl. Instrum. Methods Phys. Res. A 940 (2019) 215–222. [7] R. Britton, A. Davies, Nucl. Instrum. Methods Phys. Res. A 786 (2015) 12–16. [8] J. Burnett, A. Davies, J. McLarty, J. Radioanal. Nucl. Chem. 298–2 (2013) 987–992. [9] J. Burnett, Nucl. Instrum. Methods Phys. Res. A 944 (2019) 162473. [10] R. Brun, F. Rademakers, Nucl. Instrum. Methods Phys. Res. A 389 (1997) 81–86. [11] M. Bhat, Evaluated nuclear structure data file (ENSDF), Nucl. Data Sci. Technol. 81 (1992) 7–821. [12] W. Gelletly, J. Eberth, Lect. Notes Phys. 700 (2006) 79–117. [13] M. Riley, J. Simpson, Nuclear 𝛾-spectroscopy and the 𝛾-spheres, in: KGaA (Ed.), Digital Encyclopedia of Applied Physics, Wiley-VCH Verlag GmbH & Co, 2014. [14] J.N. Ullom, D.A. Bennett, Supercond. Sci. Technol. 28 (8) (2015) 084003. [15] W.B. Doriese, et al., Rev. Sci. Instrum. 88 (5) (2017) 053108.
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